The use of a low cost bipolar junction transistor (BJT) for the primary switch in an offline power converter is desirable, since it provides both high breakdown voltage and low on-state voltage. However, a disadvantage suffered by the BJT when compared to a voltage-controlled MOSFET, IGBT, etc., is the comparatively large reservoir of charge needed during the start-up procedure.
Typically a reservoir capacitor CAux is charged from the rectified mains input to an initialisation voltage; it then provides all of the charge, as an auxiliary supply, to both power the IC controller and drive the BJT, until a bias winding voltage rises sufficiently to take over these roles. If the initialisation charge in CAux does not power the IC and BJT for long enough to raise the bias winding voltage to its operational level, the power converter will fail to start. Loads having a high input capacitance are a particular challenge, as they present an effective short circuit in a ‘cold start’ situation; more switching cycles are therefore required to raise the output and bias winding voltages. A BJT requires significantly more charge to turn on than does a voltage-controlled device, so the charge reservoir runs down more quickly. A larger reservoir capacitance requires a higher start-up current, which can increase power consumption. Operating with a higher reservoir voltage provides more charge to support start-up for a given reservoir capacitance, and also provides more headroom for driving devices in the IC. However it also increases power consumption due to dissipation losses.
Other operating conditions may similarly raise technical issues. The bias winding voltage is generally closely related to the converter output voltage, so when the output voltage is low there may be insufficient controller IC bias power. This may occur, for converters operating in constant current or constant power output modes, when a low impedance load causes the output voltage to drop. Although this problem does occur for MOSFET-based converters, the large current required by a BJT exacerbates it considerably.
Thus, with regard to an offline power converter employing for example a BJT as primary switch, improvements are desired in relation to efficient operation and/or generally in relation the effective supply of charge to the controller IC by conventional means.
In view of the above, the field of SMPCs continues to provide a need for improved control over the supply of operating current to IC controllers of power converters, for example cascode power converters comprising bipolar or field effect primary switches.
For use in understanding the present invention, the following disclosures are referred to:                U.S. Pat. No. 7,636,246 (Inventors Huynh et al, assignee Active-Semi, Inc.), corresponding to US20070891397 published Feb. 12, 2009;        UCC28610 datasheet, available from Texas Instruments at http://www.ti.com/product/ucc28610; and        THX202H datasheet, available from: http://bbs.dianyuan.com/bbs/u/55/1330441183681758.pdf;        